US6573575B1 - DRAM MOS field effect transistors with thresholds determined by differential gate doping - Google Patents
DRAM MOS field effect transistors with thresholds determined by differential gate doping Download PDFInfo
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- US6573575B1 US6573575B1 US09/684,840 US68484000A US6573575B1 US 6573575 B1 US6573575 B1 US 6573575B1 US 68484000 A US68484000 A US 68484000A US 6573575 B1 US6573575 B1 US 6573575B1
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- gate electrode
- impurity concentration
- polysilicon film
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10B—ELECTRONIC MEMORY DEVICES
- H10B12/00—Dynamic random access memory [DRAM] devices
- H10B12/01—Manufacture or treatment
- H10B12/09—Manufacture or treatment with simultaneous manufacture of the peripheral circuit region and memory cells
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10B—ELECTRONIC MEMORY DEVICES
- H10B12/00—Dynamic random access memory [DRAM] devices
- H10B12/01—Manufacture or treatment
- H10B12/02—Manufacture or treatment for one transistor one-capacitor [1T-1C] memory cells
- H10B12/05—Making the transistor
Definitions
- the present invention relates to a semiconductor device such as DRAM (dynamic random access memory) having transistors having different threshold voltages, and a method for fabricating the same.
- DRAM dynamic random access memory
- the DRAM comprises a memory cell array composed of a number of memory cells for storing information, and a peripheral circuit including a decoding circuit for selecting a memory cell, the memory cells and the peripheral circuit being formed on the same semiconductor substrate.
- a memory-logic mixed construction has been adopted in which the DRAM and a logic circuit developed by a user are formed on the same semiconductor substrate.
- the peripheral circuit includes the logic circuit.
- a memory cell includes a capacitor accumulating a signal charge for storing information, and a transistor functioning as a switching device for accumulating the signal charge into the capacitor and for reading out the signal charge accumulated in the capacitor.
- the transistor is ordinarily constituted of a FET (field effect transistor) having a MOS (metal oxide semiconductor) structure or a MIS (metal insulator semiconductor), because these transistors are convenient for realizing a high integration density.
- the peripheral circuit is constituted of FETs having the same structure as that of the transistor used in the memory cell, in order to make a fabricating processing in common with that for the memory cell. In the following, explanation will be made under the assumption that an n-channel MOS FET is used as transistors for the memory cells and transistors for the peripheral circuit.
- a recent MOSFET has a gate electrode (word line) formed of an n + polysilicon film or alternatively a multilayer film composed of a metal silicide and the n + polysilicon film (polycide film). This is because the work function of the n + silicon (Si) of the gate electrode is lower than the work function of a p-type silicon in a channel region by about 1V, so that it is possible to easily set the threshold of the FET at a low value.
- the drain current flowing when the gate voltage is 0V is called a subthreshold leak current If there is the subthreshold leak current in the FET used in the DRAM memory cell, the signal charge accumulated in the memory cell capacitor is discharged so that it becomes impossible to hold the signal charge. Therefore, the FET used in the DRAM memory cell is required to have as a small subthreshold leak current as possible.
- the threshold is set to be high.
- the threshold is set to be relatively high (on the order of 1.2V) in order to simultaneously prevent a malfunction caused by noises or others.
- the threshold is set to be relatively low (on the order of 0.4 to 0.6V) because reduction of an on-resistance to realize a high speed operation and reduction of a power consumption, are in preference to the reduction of the subthreshold leak current.
- the threshold voltage was changed by making the impurity concentration of the channel region in the FET used in the peripheral circuit, different from the impurity concentration of the channel region in the FET of the memory cell.
- the impurity concentration of the channel region in the FET of the memory cell is set to be five to ten times the impurity concentration of the channel region in the FET used in the peripheral circuit.
- FIG. 4 there is shown a partial diagrammatic section view of the DRAM for illustrating one example of the prior art semiconductor device.
- the prior art DRAM includes a memory cell FET and a peripheral circuit FET which are formed on a p-type semiconductor substrate 1 and which are isolated from each other by a field oxide film 2 formed of an insulating film such as silicon dioxide (SiO 2 ), in a shallowly embedded structure at a principal surface of the substrate.
- a field oxide film 2 formed of an insulating film such as silicon dioxide (SiO 2 ), in a shallowly embedded structure at a principal surface of the substrate.
- the memory cell is composed of an FET and a capacitor.
- Each of the memory cell FET and the peripheral circuit FET includes a drain region 3 and a source region 4 constituted of an n + diffused layer formed at the principal surface of the substrate, a channel region 5 or 15 formed between the drain region 3 and the source region 4 , a gate insulator film 6 formed on the principal surface of the substrate, directly above the channel region 5 or 15 , and constituted of an insulating film such as silicon dioxide (SiO 2 ), and agate electrode formed of a n + polysilicon film 8 and a metal silicide film 9 stacked on the gate insulator film 6 .
- the capacitor includes a capacitor lower electrode 11 connected to the drain 3 of the memory cell FET and becoming as one electrode of a pair of electrodes of a capacitor for holding information, a capacitor upper electrode 12 becoming as the other electrode of the capacitor, and a capacitor dielectric film 13 sandwiched between the capacitor lower electrode 11 and the capacitor upper electrode 12 .
- each of he capacitor lower electrode 11 and the capacitor upper electrode 12 is formed of an n + polysilicon
- the capacitor dielectric film 13 is formed of a silicon nitride film (Si 3 N 4 ).
- the memory cell FET and the peripheral circuit FET have the same structure, but the prior art DRAM was so formed that the impurity concentration of the channel region 15 in the peripheral circuit FET is lower than the impurity concentration of the channel region 5 in the memory cell FET. (Difference in impurity concentration is indicated in FIG. 4 by difference in thickness of the dotted line in the channel region.)
- FIGS. 5A to 5 F are diagrammatic sectional views of the DRAM for illustrating various steps of the semiconductor device fabricating process.
- a silicon nitride film is formed on a principal surface of a p-type semiconductor substrate 1 , and is patterned to have a predetermined shape.
- the substrate exposed in an opening of the patterned silicon nitride film is selectively oxidized to form a field oxide film 2 having the thickness on the order of 300 nm, which constitutes an inactive region for a device isolation, as shown in FIG. 5 A.
- a pad oxide film 20 having the thickness on the order of 20 nm is formed on the whole of the principal surface of the substrate by means of a thermal oxidation.
- boron (B) is ion-implanted through the pad oxide film 20 with an energy of 30 KeV and a dose of 1 to 2 ⁇ 10 12 atoms/cm 2 (FIG. 5 B).
- a photoresist 21 is formed on the substrate to cover a region in which the peripheral circuit.
- FET is formed
- boron (B) is further ion-implanted into a region in which the memory cell FET is formed, with an energy of 30 KeV and a dose of 7 to 8 ⁇ 10 12 atoms/cm 2 (FIG. 5 C).
- the impurity concentration of the channel region 5 in the memory cell FET and the impurity concentration of the channel region 15 in the peripheral circuits FET are made different from each other
- the order of the ion implantation shown in FIG. 5 B and the ion implantation shown in FIG. 5C can be exchanged.
- a gate oxide film 6 having the thickness on the order of 10 nm is formed on the principal surface of the substrate by means of a thermal oxidation.
- an n + polysilicon film 8 having a high impurity concentration (on the order of 2 to 3 ⁇ 10 20 atoms/cm 3 ) and a thickness on the, order of 100 nm, and a refractory metal silicide film (WSi) 9 having the thickness on the order of 100 nm, which are used to form a gate electrode are deposited on the gate oxide film 6 by means of a CVD (chemical vapor deposition) process or a reactive sputtering (FIG. 5 D).
- the n + polysilicon film 8 and the metal silicide film 9 are patterned by a lithography, to form a gate electrode of the memory cell FET and a gate electrode of the peripheral circuit FET.
- arsenic (As) is ion-implanted with a dose of 1 ⁇ 10 16 atoms/cm 2 , and then, is thermally diffused to form an n + diffused layer (FIG. 5 E).
- an interlayer insulating film 10 having the thickness on the order of 500 nm is deposited on the whole of the principal surface of the substrate to cover the gate electrodes.
- a contact hole is formed in the interlayer insulating film 10 to reach the drain region 4 of the memory cell FET, and a phosphorus (P) doped n + polysilicon is deposited to fill up the contact hole and to form a capacitor lower electrode 11 .
- a capacitor dielectric film 13 having the thickness on the order of 8 nm is deposited on the capacitor lower electrode 11 .
- a phosphorus (P) doped n + polysilicon is deposited on the capacitor dielectric film 13 to have a thickness on the order of 200 nm, thereby to form a capacitor upper electrode 12 .
- an interlayer insulating film 10 is deposited on the whole of the principal surface of the substrate to cover the capacitor upper electrode 12 (FIG. 5 F).
- the interlayer insulating film 10 shown as being formed of only one layer, but is actually formed of a plurality of layers as seen from the above mentioned process.
- the source-drain subthreshold leak current can be reduced by increasing the impurity concentration of the channel region 5 .
- another problem occurs in which a leak current increases between the substrate 1 of the p-type semiconductor and the source region 4 or the drain region 3 of the n-type semiconductor, with the result that the performance of holding the signal charge by the capacitor drops.
- the FET microminiaturized with an increase of the memory capacity as in a recent DRAM since a source-drain distance gradually becomes short, it is necessary to increase the impurity concentration of the channel region 5 , with the result that the leak current in the PN junction increases more or more.
- JP-A-04-357865, JP-A-09-036318 and JP-A-11-026711 propose a technology of using a p + polysilicon film in the gate electrode of a FET whose threshold is desired to be elevated.
- FIGS. 6A and 6B are energy band diagrams for illustrating an energy level of the gate electrode and the semiconductor substrate in the FET in the prior art semiconductor device.
- FIG. 6A illustrate an energy band diagram when the gate electrode is formed of the n + polysilicon film
- FIG. 6B illustrate an energy band diagram when the gate electrode is formed of the p + polysilicon film.
- the solid lines indicate the energy level when the gate voltage Vg is the threshold V T .
- the channel region is formed within the body of the semiconductor substrate, not in the neighborhood of the surface of the semiconductor substrate, as shown in FIG. 7 (which illustrates a condition in that the channel elongates from the drain by action of the voltage applied between the source and the drain).
- the subthreshold factor of the FET lowers, with the result that controllability of the source-drain current by the gate voltage is deteriorated, and the subthreshold leak current increases.
- boron (B) included in the p + polysilicon film is liable to be captured into SiO 2 of the gate oxide film in a heat treatment step, in comparison with phosphorus (p) included in the n + polysilicon film. Therefore, the threshold voltage of the PET changes, and the insulating performance of the gate insulating film lowers, so that reliability drops.
- the gate electrode of the memory cell FET is formed by using the p + polysilicon film and the gate electrode of the peripheral circuit FET is formed by using the n + polysilicon film
- the n + polysilicon film was formed on the p + polysilicon film
- the phosphorus (P) in the n + polysilicon film is diffused into the p + polysilicon film in a heat treatment performed at a later step, so that a region including only a reduced amount of impurity (phosphorus) is formed, with the result that a so called PIN structure is formed.
- a metal silicide for example WSi
- the metal silicide does not function as a barrier.
- a metal material such as aluminum (Al).
- Al aluminum
- the metal is inferior to the n + polysilicon in the property of filling up the contact hole, it is necessary to enlarge the contact hole. This cannot be adopted in a recent highly integrated and microminiaturized semiconductor device.
- Another object of the present invention is to provide a semiconductor device and a method for fabricating the same, capable of elevating the threshold voltage, without various problems such as an increased leak current caused by increasing the impurity concentration of the channel region, the lowered subthreshold factor caused by using the p + polysilicon film in the gate electrode, the deteriorated insulating performance of the gate oxide film, the increased number of fabricating steps, or the dropped reliability of the transistor.
- a semiconductor device including a first n-channel transistor having a first threshold voltage and a second n-channel transistor having a second threshold voltage, the first threshold voltage being higher than the second threshold voltage, each of the first and second n-channel transistors having a gate electrode including an n-type polysilicon film, wherein the n-type polysilicon film included in the gate electrode of the first n-channel transistor has a first impurity concentration lower than a second impurity concentration in the n-type polysilicon film included in the gate electrode of the second n-channel transistor.
- the first impurity concentration is in the range of 1 ⁇ 2 to ⁇ fraction (1/10) ⁇ of the second impurity concentration.
- the first impurity concentration can be in the range of not less than 1 ⁇ 10 19 atoms/cm 3 but not greater than 1 ⁇ 10 20 atoms/cm 3 .
- a channel region of the first n-channel transistor can have an impurity concentration equal to that in a channel region of the second n-channel transistor.
- the impurity concentration of the n-type polysilicon film included in the gate electrode of the first n-channel transistor having a relatively high threshold is lower than the impurity concentration of the n-type polysilicon film included in the gate electrode of the second n-channel transistor having a relatively low threshold.
- the difference in work function between the gate electrode and a p-type semiconductor substrate becomes small. Therefore, it is possible to elevate the threshold voltage of the transistor without increasing the impurity concentration in the channel region, similarly to the prior art example having the gate electrode formed of the p + polysilicon film.
- a method for fabricating a semiconductor device including a first n-channel transistor having a first threshold voltage and a second n-channel transistor having a second threshold voltage lower than the first threshold voltage, each of the first and second n-channel transistors having a gate electrode including an n-type polysilicon film, the method including the step,
- a method for fabricating a semiconductor device including a first n-channel transistor having a first threshold voltage and a second n-channel transistor having a second threshold voltage lower than the first threshold voltage, each of the first and second n-channel transistors having a gate electrode including an n-type polysilicon film, the method including the step,
- a channel region of the first n-channel transistor and a channel region of the second n-channel transistor can be simultaneously formed by the same ion implantation step.
- the impurity concentration of the gate electrode formed of the n-type polysilicon film in the first n-channel transistor can be made different from the impurity concentration of the gate electrode formed of the n-type polysilicon film in the second n-channel transistor, without substantially increasing the number of fabricating steps in comparison with the prior art.
- FIG. 1 is a partial diagrammatic section view of the DRAM for illustrating one embodiment of the semiconductor device in accordance with the present invention
- FIG. 2 is an energy band diagram for illustrating an energy level of the gate electrode and the semiconductor substrate in the memory cell FET in the semiconductor device shown in FIG. 1;
- FIGS. 3A to 3 F are diagrammatic sectional views of the DRAM for illustrating various steps of the process for fabricating the semiconductor device shown in FIG. 1;
- FIG. 4 is a partial diagrammatic section view of the DRAM for illustrating one example of the prior art semiconductor device
- FIGS. 5A to 5 F are diagrammatic sectional views of the DRAM for illustrating various steps of the process for fabricating the semiconductor device shown in FIG. 4;
- FIGS. 6A and 6B are energy band diagrams for illustrating an energy level of the gate electrode and the semiconductor substrate in the FET in the prior arts semiconductor device.
- FIG. 7 illustrates a condition in that the channel elongates from the drain by action of the voltage applied between the source and the drain in the memory cell FET when the gate electrode is formed of the p + polysilicon.
- FIG. 1 there is shown a partial diagrammatic section view of the DRAM for illustrating one embodiment of the semiconductor device in accordance with the present invention.
- FIG. 2 is an energy band diagram for illustrating an energy level of the gate electrode and the semiconductor substrate in the memory cell FET in the semiconductor device shown in FIG. 1 .
- the solid lines indicate the energy level when the gate voltage Vg is the threshold V T .
- the DRAM of this embodiment is so configured that a gate electrode of the memory cell FET is formed of a multilayer film composed of an n-type polysilicon film and a refractory metal silicide film, the n-type polysilicon film having an impurity concentration lower than that of the n-type polysilicon film included in the multilayer gate electrode of the peripheral circuit FET.
- the n-type polysilicon film included in the multilayer gate electrode of the memory cell FET is shown as an n ⁇ polysilicon film 7 .
- the channel region 5 of the memory cell FET and the channel region 5 of the peripheral circuit FET have an equal impurity concentration. Since the other construction is the same as that of the prior art example shown in FIG.
- the equal impurity concentration in the channel regions 5 of the memory cell FET and the peripheral circuit FET is illustrated by equalizing the thickness of the dotted lines depicted in the respective channel regions in FIG. 1 .
- the different impurity concentrations in the n-type polysilicon film of the gate electrode are illustrating by changing the thickness of the vertical solid lines depicted in the respective gate electrode n-type polysilicon films in FIG. 1 .
- the difference in work function between the gate electrode and a p-type semiconductor substrate becomes small, as seen from FIG. 2 . Therefore, it is possible to elevate the threshold voltage of the FET without increasing the impurity concentration in the channel region, similarly to the prior art example having the gate electrode formed of the p + polysilicon film
- the threshold voltage can be easily elevated without various problems such as an increased leak current caused by increasing the impurity concentration of the channel region, the lowered subthreshold factor caused by using the p + polysilicon film in the gate electrode, the deteriorated insulating performance of the gate oxide film, the increased number of fabricating steps, or the dropped reliability of the transistor.
- the impurity concentration of the n-type polysilicon film included in the gate electrode of the memory cell FET is preferred to be in the range of 1 ⁇ 2 to ⁇ fraction (1/10) ⁇ of the impurity concentration (2 to 3 ⁇ 10 20 atoms/cm 3 ) of the n-type polysilicon film included in the gate electrode of the peripheral circuit FET. More specifically, it is preferred to be in the range of not less than 1 ⁇ 10 19 atoms/cm 3 but not greater than 1 ⁇ 10 20 atoms/cm 3 . The reason for this is that: If the impurity concentration of the n-type polysilicon film is greater than 1 ⁇ 10 20 atoms/cm 3 , a substantial difference does not appear in the threshold voltage of the FET. If the impurity concentration is less than 1 ⁇ 10 19 atoms/cm 3 , the n-polysilicon film of the gate electrode becomes a depletion condition, so that even if the gate voltage is applied the FET is no longer turned on.
- FIGS. 3A to 3 F are diagrammatic sectional views of the DRAM for illustrating various steps of the process for fabricating the semiconductor device shown in FIG. 1 .
- a silicon nitride film is formed on a principal surface of a p-type semiconductor substrate 1 , and is patterned to have a predetermined shape.
- the substrate exposed in an opening of the patterned silicon nitride film is selectively oxidized to form a field oxide film 2 having the thickness on the order of 300 nm, which constitutes an inactive region for a device isolation, as shown in FIG. 3 A.
- the device isolation region may be a shallow trench isolation (STI).
- a pad oxide film 20 having the thickness on the order of 20 nm is formed on the whole of the principal surface of the substrate by means of a thermal oxidation.
- boron (B) is ion-implanted through the pad oxide film 20 with an energy of 30 KeV and a dose of 1 to 2 ⁇ 10 12 atoms/cm 2 (FIG. 3 B).
- a gate oxide film 6 having the thickness on the order of 10 nm is formed on the principal surface of the substrate by means of a thermal oxidation.
- a photoresist 22 is formed on the n ⁇ polysilicon film 7 to cover an area excluding a region in which the peripheral circuit FET is formed, and phosphorus (P) is ion-implanted into the region in which the peripheral circuit FET is formed, with an energy of 30 KeV and a dose of 0.5 to 1 ⁇ 10 16 atoms/cm 2 (FIG. 3 C), so that n ⁇ polysilicon film 7 is converted into an n + polysilicon film 8 in the region in which the peripheral circuit FET is formed.
- P phosphorus
- the impurity concentration of the n-type polysilicon film included in the gate electrode of the peripheral circuit FET is made different from the impurity concentration of the n-type polysilicon film included in the gate electrode of the memory cell FET.
- the n ⁇ polysilicon film 7 , die n + polysilicon film 8 and the metal silicide film 9 are patterned by a lithography, to form a gate electrode of the memory cell FET and a gate electrode of the peripheral circuit FET, respectively.
- arsenic (As) is ion-implanted with a dose of 1 ⁇ 10 16 atoms/cm 2 , and then, is thermally diffused to form an n + diffused layer (FIG. 3 E).
- an interlayer insulating film 10 having the thickness on the order of 500 nm is deposited on the whole of the principal surface of the substrate to cover the gate electrodes.
- a contact hole is formed in the interlayer insulating film 10 to reach the drain region 4 of the memory cell FET, and a phosphorus (P) doped n + polysilicon is deposited to fill up the contact hole and to form a capacitor lower electrode 11 .
- a capacitor dielectric film 13 having the thickness on the order of 8 nm is deposited on the capacitor lower electrode 11 .
- a phosphorus (P) doped n + polysilicon is deposited on the capacitor dielectric film 13 to have a thickness on the order of 200 nm, thereby to form a capacitor upper electrode 12 .
- an interlayer insulating film 10 is deposited on the whole of the principal surface of the substrate to cover the capacitor upper electrode 12 (FIG. 3 F). Incidentally, in FIGS. 1 and 3F, the interlayer insulating film 10 shown as being formed of only one layer, but is actually formed of a plurality of layers as seen from the above mentioned process.
- the impurity concentration of the gate electrode formed of the n-type polysilicon film in the memory cell FET can be made different from the impurity concentration of the gate electrode formed of the n-type polysilicon film in the peripheral circuit FET, without substantially increasing the number of fabricating steps in comparison with the prior art.
- the present invention is applied to the DRAM in which a memory cell FET having a high threshold voltage and a peripheral circuit FET having a low threshold voltage are formed on a single semiconductor substrate.
- a memory cell FET having a high threshold voltage and a peripheral circuit FET having a low threshold voltage are formed on a single semiconductor substrate.
- the present invention can be applied to a semiconductor device having a plurality of transistors (particularly, n-channel MOS FETs) having different threshold voltages.
- the impurity concentration of the n-type polysilicon film included in the gate electrode of the first n-channel transistor having a relatively high threshold is lower than the impurity concentration of the n-type polysilicon film included in the gate electrode of the second n-channel transistor having a relatively low threshold.
- the difference in work function between the gate electrode and a p-type semiconductor substrate becomes small. Therefore, it is possible to elevate the threshold voltage of the transistor without increasing the impurity concentration in the channel region, similarly to the prior art example having the gate electrode formed of the p + polysilicon film.
- the threshold voltage can be easily elevated without various problems such as an increased leak current caused by increasing the impurity concentration of the channel region, the lowered subthreshold factor caused by using the p + polysilicon film in the gate electrode, the deteriorated insulating performance of the gate oxide film, the increased number of fabricating steps, or the dropped reliability of the transistor.
Abstract
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JP11-285483 | 1999-10-06 | ||
JP28548399A JP2001110908A (en) | 1999-10-06 | 1999-10-06 | Semiconductor device and its manufacturing method |
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US09/684,840 Expired - Fee Related US6573575B1 (en) | 1999-10-06 | 2000-10-06 | DRAM MOS field effect transistors with thresholds determined by differential gate doping |
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US20040150020A1 (en) * | 2002-08-27 | 2004-08-05 | Elpida Memory, Inc. | Semiconductor device and method of producing the same |
US20040166623A1 (en) * | 2003-02-20 | 2004-08-26 | Lee Chang Yeol | Method for reducing poly-depletion in dual gate CMOS fabrication process |
US20060030090A1 (en) * | 2004-08-06 | 2006-02-09 | Wei-Pang Huang | Thin film devices for flat panel displays and methods for forming the same |
US20120056272A1 (en) * | 2009-06-17 | 2012-03-08 | Panasonic Corporation | Semiconductor device |
US8860223B1 (en) * | 2010-07-15 | 2014-10-14 | Micron Technology, Inc. | Resistive random access memory |
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US20120056272A1 (en) * | 2009-06-17 | 2012-03-08 | Panasonic Corporation | Semiconductor device |
US8860223B1 (en) * | 2010-07-15 | 2014-10-14 | Micron Technology, Inc. | Resistive random access memory |
US9570681B2 (en) | 2010-07-15 | 2017-02-14 | Micron Technology, Inc. | Resistive random access memory |
Also Published As
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GB0024565D0 (en) | 2000-11-22 |
GB2361357A (en) | 2001-10-17 |
JP2001110908A (en) | 2001-04-20 |
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